Master Reset a Series 60 phone

Series 60 phones, like the Nokia 6600, have two reset sequences — a Normal Reset and a Deep Reset.

Normal Reset -- *#7780# -- Restores all settings from the ROM but preserves your data, like your
images, applications, etc.

Deep Reset -- *#7370# -- Formats your C: completely and restores the phone to the factory defaults.
All files and data on C: will be lost.

The phone will ask you for a confirmation when you ask for a reset and you need to enter your security code, which is 12345 by default.

Note that neither the Normal Reset nor the Deep Reset touches your memory card.